As worldwide energy demand continues to increase and reserves of fossil fuels shrink, the diversification of energy sources is increasingly important. Biodiesel is one way to achieve diversification. Biodiesel is a generic name for mono-alkyl esters of long-chain fatty acids derived from renewable lipid sources such as vegetable oils, animal fats, or used cooking oils and fats. Biodiesel fuels have many names, depending on the feedstocks used to produce them, and biodiesel feedstocks vary widely in their fatty acid compositions (chain length and saturation). Non-limiting examples of some common feedstocks used in the production of biodiesel include fatty acid methyl ester (FAME), rapeseed methyl ester (RME), used vegetable oil methyl ester (UVOME), soybean oil methyl ester (SOME) or palm oil methyl ester (POME).
Biodiesel is typically produced by the reaction of a vegetable oil or an animal fat with an alcohol, such as methanol, in the presence of a catalyst to yield methyl esters (the biodiesel) and glycerine. The most commonly used catalyst, potassium hydroxide, is used in transesterification of a wide range of oils and fats, from vegetable to animal, from virgin to used, including those with the highest acid contents. The thus produced biodiesel can be distilled to remove excess alcohols and other impurities. Other methods for production of biodiesel are known.
As is known in the art, biodiesel are subject to oxidative degradation. Oxidation of biodiesel by contact with air and metal surfaces results in the formation of hydroperoxides. These induce free-radical chain reactions that lead to decomposition into low-molecular-weight, highly oxidized species (aldehydes, ketones, acids) and high-molecular-weight polymeric materials (gums). These gums tend to cause poor combustion and other engine problems such as deposits on injectors and pistons. The presence of high-molecular weight, insoluble gums generally leads to fuel-filter plugging.
The oxidation stability of biodiesel can be increased by the addition of antioxidants. However, given the relative youth of biodiesel fuels as compared to hydrocarbon fuels, antioxidant technology for biodiesel is not as well-developed.
Thus, there is a need for improved antioxidant compositions for use in biodiesel fuels and for biodiesel compositions comprising such antioxidant compositions that are economically suited for commercial use.